nbd_opt_starttls3

Usage

 #include <libnbd.h>
 int nbd_opt_starttls (
       struct nbd_handle *h
     );

Overview

Request that the server initiate a secure TLS connection, by sending "NBD_OPT_STARTTLS". This can only be used if nbd_set_opt_mode(3) enabled option mode; furthermore, if you use nbd_set_tls(3) to request anything other than the default of "LIBNBD_TLS_DISABLE", then libnbd will have already attempted a TLS connection prior to allowing you control over option negotiation. This command is disabled if nbd_supports_tls(3) reports false.

This function is mainly useful for integration testing of corner cases in server handling; in particular, misuse of this function when coupled with a server that is not careful about resetting stateful commands such as nbd_opt_structured_reply(3) could result in a security hole (see CVE-2021-3716 against nbdkit, for example). Thus, when security is a concern, you should instead prefer to use nbd_set_tls(3) with "LIBNBD_TLS_REQUIRE" and let libnbd negotiate TLS automatically.

This function returns true if the server replies with success, false if the server replies with an error, and fails only if the server does not reply (such as for a loss of connection, which can include when the server rejects credentials supplied during the TLS handshake). Note that the NBD protocol documents that requesting TLS after it is already enabled is a client error; most servers will gracefully fail a second request, but that does not downgrade a TLS session that has already been established, as reported by nbd_get_tls_negotiated(3).

Return value

This call returns a boolean value.

Errors

On error -1 is returned.

Refer to "ERROR HANDLING" in libnbd(3) for how to get further details of the error.

The following parameters must not be NULL: "h". For more information see "Non-NULL parameters" in libnbd(3).

Handle state

nbd_opt_starttls can be called when the handle is in the following state:

 ┌─────────────────────────────────────┬─────────────────────────┐
 │ Handle created, before connecting   │ ❌ error                │
 │ Connecting                          │ ❌ error                │
 │ Connecting & handshaking (opt_mode) │ ✅ allowed              │
 │ Connected to the server             │ ❌ error                │
 │ Connection shut down                │ ❌ error                │
 │ Handle dead                         │ ❌ error                │
 └─────────────────────────────────────┴─────────────────────────┘

Version

This function first appeared in libnbd 1.16.

If you need to test if this function is available at compile time check if the following macro is defined:

 #define LIBNBD_HAVE_NBD_OPT_STARTTLS 1